1. Field of the Invention
This invention relates generally to radio frequency receivers, and more specifically to reducing direct current (DC) offset in a direct conversion receiver.
2. Related Art
Direct current (DC) offset in a zero-IF, or direct conversion, receiver is a known concern that normally should be dealt with for such receiver to operate properly. DC offset may be viewed as an error term that is added to, and becomes part of, a received signal. DC offset can result from mismatches within a receiver lineup, e.g., mismatches in quadrature mixers, amplifiers and/or filters. These mismatches may be more pronounced in receiver lineups that are implemented substantially in integrated circuit form because techniques, such as suitable blocking capacitors, are not readily available to mitigate the DC offset.
In one known method, a DC offset calibration is performed with a first-order mixed-signal control loop in order to correct for any DC offset. The control loop increases the usable dynamic range through an analog baseband signal path of the direct conversion receiver.
FIG. 1 is a block diagram of a known direct conversion receiver 100 that includes a DC offset correction system 135. A received signal at a radio frequency (RF) from an antenna 109 is coupled via a transmit/receive (T/R) switch 111 to a low noise amplifier (LNA) 113. The LNA 113 is a switchable gain amplifier that amplifies the RF signal. The output of the LNA 113 is coupled to a mixer 105 via a capacitor 114. The receiver 100 includes a local oscillator (LO) 117 coupled to a quadrature generator 118. The quadrature generator 118 phase shifts the signal from the LO 117 by 90°. The quadrature generator 118 provides an oscillator signal to the in-phase (I-phase) mixer 105 and an oscillator signal phase shifted by 90° to a quadrature-phase (Q-phase) mixer (not shown). In FIG. 1, only the I-phase receive path and the I-phase mixer 105 are shown. The mixer 105 is driven by a signal from the quadrature generator 118 having a frequency that is nominally equal to the frequency of the RF signal, and thus the desired output signal from the mixer is nominally at 0-Hz. A front end of the front end of the receiver 100 includes the T/R switch 111, the LNA 113 and the quadrature generator 118.
The receiver 100 includes an amplifier 119, which is a variable gain amplifier with a gain that can be changed via a control signal 120. The amplifier 119 is coupled to a lowpass filter 121 with a bandwidth selected to correspond to the bandwidth of the received RF signal. The output from lowpass filter 121 is coupled to an amplifier 123 having a variable gain that can be changed via a control signal 124. The output from the amplifier 123 is coupled to another lowpass filter 125. The output of lowpass filter 125 is coupled to an amplifier 127 that also has a variable gain. The output of the amplifier 127 drives an analog-to-digital converter (ADC) 129, to provide I-samples at the output of the ADC. The I-samples are fed to a DC offset correction system 135. The DC offset correction system 135 operates to detect and determine the level of any DC offset in the receiver 100, and to provide signals via a digital-to-analog converter (DAC) 137 to an input of the lowpass filter 125, so as to reduce the DC offset at the input of the ADC 129.
The receiver 100 includes an automatic gain control (AGC) 143 that provides a control signal at 149 to control the T/R switch 111 to change between receive and transmit modes of operation. The AGC 143 also provides a control signal at 115 to control gain of the LNA 113 during open loop calibration, i.e., during normal receive operation. The AGC 143 does not control gain of the LNA 113 during closed loop calibration. The AGC 143 also generates a gain control signal at 145 that is coupled to a decoder 147. The decoder 147 performs a mapping function to convert the gain control signal at 145 to respective, control signals at 120, 124 and 128. The decoder 147 provides the control signals at 120, 124 and 128 based on a received signal strength, gain status, and non-uniform gain control steps of the amplifiers 119, 123 and 127, such that an appropriately limited signal amplitude is presented to the ADC 129.
The DC offset correction system 135 includes a register bank 150 that is configured to store a plurality of offset data corresponding to a plurality of gain settings for the receiver 100, i.e., offset data corresponding to each of a plurality of gain settings for the amplifiers 119, 123 and 127. The DC offset correction system 135 provides an offset signal at 154. The offset signal at 154 is a digital signal that is delayed at a delay stage 156, and then applied to the DAC 137 with a resultant analog signal coupled to an adder 158. The analog signal is combined with, i.e., subtracted from, the received signal and thereby shifts the received signal so as to reduce or substantially eliminate any DC offset at the input to the ADC 129. The DC offset correction system 135 includes a decimation filter 168 that is coupled to the output of the ADC 129. The output of the decimation filter 168 is coupled to a decimator 170. The signal from the decimator 170 is coupled to a gain stage 174. The output from the gain stage 174 is applied to a loop filter 176 that acts as an integrator. The loop filter 176 comprises an adder 180, a multiplexer 182 and a one clock cycle delay element (z−1) 184. The output of the loop filter 176 is coupled to a decimator 186. Additionally, the DC offset correction system 135 includes control circuitry 190 that is coupled to a non-volatile memory 196, to the loop filter 176, and to the register bank 150.
The DCOC correction system 135 is designed to operate in the presence of only noise, i.e., no signal. A signal should not be present at the antenna 109 while performing the DCOC calibration in order to avoid having the DCOC calibration algorithm settle on an incoming RF signal. Because the DCOC correction system 135 performs DCOC calibration in the field, the DCOC calibration is performed with the front end of the receiver 100 disabled, which means the antenna 109 is disconnected from the receiver by the T/R switch 111 and the LNA 113 is disabled. Disadvantageously, the DCOC correction system 135 cannot perform DCOC calibration when the LNA 113 is enabled because an incoming signal would adversely affect the DCOC calibration.
Typically, most of the components shown in FIG. 1 are located on a single integrated circuit chip 101; however, the antenna 109, the T/R switch 111 and the LNA 113 are located off the chip 101.
It is desirable to prevent any self-mixing at the mixer 105 that results in a DC offset at the output of the mixer. Self-mixing at the mixer 105 occurs as a result of leakage of the LO signal. Leakage of the LO signal occurs because of a lack of complete isolation between the LO 117 and the LNA 113. Any DC offset at the output of the mixer 105 is amplified by the amplifiers 119, 123 and 127, thus saturating the baseband lineup.
The mixing of signals having identical frequency characteristic results in a DC offset at the output of the mixer 105. For example, leakage of the input RF signal may disadvantageously appear at the input port of the mixer 105 to which the LO signal is inputted. The leakage components are driven to the mixer 105 together with the LO signal. Consequently, the input RF signal is mixed with the leakage components of itself at mixer 105.
Similarly, leakage of the LO signal may disadvantageously appear at the input port of the mixer 105 to which input RF signal is inputted. The leakage components are driven to the mixer 105 together with the input RF signal. Consequently, the LO signal is mixed with the leakage components of itself at the mixer 105. For example, LO leakage occurs when the LO signal cos ωLOt from the LO 117 is inputted into the mixer 105 at the input port to which input RF signal is inputted. The leakage signal is self-mixed with the LO signal, as shown in the following equation, and, as a result, a DC offset occurs.(cos ωLOt)(cos ωLOt)=½+cos 2ωLOt 
The DC offset corresponding to the ½ term in the preceding equation causes one or more of the amplifiers 119, 123 and 127 to saturate.
Furthermore, the LO signal may also leak from the RF signal input port of the mixer 105 and be disadvantageously reflected at the output port of the LNA 113 and/or at the antenna 109. LO leakage can occur through conductive, magnetic or RF means, including due to substrate coupling and/or bond wire coupling.
With known circuits, the self-mixing that occurs as a because of LO leakage results in additional DC offsets during open loop calibration that were not present during closed loop calibration. Such additional DC offsets (due to self-mixing) do not occur during closed loop calibration because the LNA 113 is disabled (i.e., turned off) during the closed loop calibration.
In the circuit of FIG. 1, the LNA 113 is not located on the same chip 101 as the LO 117. Although the receiver 100 functions adequately when the LNA 113 and the LO 117 are not on the same chip 101, the receiver may not operate adequately if the LNA and the LO were on the same chip.
Known DC offset correction methods and apparatus, including the circuit shown in FIG. 1, will not operate properly when LO leakage and self-mixing are present.